Lactic acid or 2-hydroxypropanoic acid is an xcex1-hydroxylated carboxylic acid which can be produced by fermenting a variety of pure (glucose, sucrose, lactose, etc.) or impure (products of the hydrolysis of starch, molasses, milk serum, etc.) carbonaceous substrates using microorganisms such as bacteria of the genera Lactobacillus, Pediococcus, Lactococcus and Streptococcus, or certain fungi such as Rhizopus Oryzae. Other routes for obtaining lactic acid, by way of chemical transformations of reagents derived from petrochemistry, such as the hydrolysis of lactonitrile, which is itself obtained starting from acetaldehyde, the chlorination and hydrolysis of propionic acid, or else via the nitration of propene, are known to the skilled person.
Lactic acid exists in two diastereoisomeric forms: L(+) and D(xe2x88x92) lactic acid, and finds novel applications every day, from the conventional use as a food preservative through to novel developments such as the synthesis of solvents, pesticides, herbicides, biodegradable polymers, etc. However, because of the increasing stringency of the quality criteria demanded and the necessity of achieving production costs which are compatible with the market, it is crucial to develop purification techniques which are efficient and inexpensive.
Lactic acid can be purified by precipitation in the form of metallic lactates, followed by a neutralization reaction using sulphuric acid (Maesato K., Komori A., Taki Chem. Co., JP 6,272,646 (Sep. 25, 1985)), or by esterification with an alcohol, distillation and hydrolysis of the ester which has been formed (Boroda T. A., Polovko V. N., Chistyakova E. A., Pishch.Prom. 1966, 4, 35-8), or by electrodialysis (Jacquement J. C., Rhone-Poulenc, DE 1,957,395 (Nov. 14, 1968)). The first of these processes suffers from the product being of poor quality and there being heavy losses of lactic acid, while the other two suffer from the fact that their cost is prohibitive. A more recent purification process consists in extracting the lactic acid by liquid/liquid extraction using at least one in water non-miscible organic solvent in the presence or absence of at least one Lewis base such as a tertiary amine. In accordance with this process, the lactic acid has to be recovered in a second step by means of carrying out a liquid/liquid back extraction. This step transfers the lactic acid back into the water. (Baniel A. M., Blumberg R., Hadju K., IMI, DE 2,329,480 (Jun. 19, 1972); Baniel A. M., Miles Lab., EP 49,429 (Oct. 06, 1980)). Finally, the lactic acid, in acid form and/or in the form of ammonium lactate or metallic lactate, can be purified by passing it through cationic and/or anionic ion exchange columns (Napierala W., Siminski M., Przem. Ferment. Rolny. 1972, 16(12), 4-10; Shkurino O. V., Dauksha V. E., Khim-Farm.Zh. 1986, 20(10), 1375-77; Maesato K., Komori A., Taki Chem.Co., JP 6,272,646 (Sep. 25, 1985); Obara H., Shimadzu corp., JP 63,188,632 (Jan. 30, 1987); Obara H., Shimadzu Corp, JP 0,191,788 (Sep. 30, 1987); Zeleneva N. A., Ivanova E. V., Karpushina I. A., Gaevskaya M. V., Teoriya I Prakitika Sorbtsionnyki Protsessov, 1982, 67-69).
It is to be noted that all these purification procedures are generally carried out starting from dilute solutions of lactic acid in water. This is justified by the actual structure of lactic acid, which carries a hydroxyl function and a carboxylic acid group at one and the same time. Thus, the bifunctionality of lactic acid is responsible for condensation reactions which generate lactoyllactic, dilactoyllactic, trilactoyllactic, . . . (n-lactoyllactic) units, which are also termed lactic acid oligomers. While these condensation or oligomerization reactions tend towards an equilibrium, the probability of their occurring increases as the concentration of the starting aqueous solution is increased (Holten C. H., xe2x80x9cLactic acid: Properties and chemistry of lactic acid and derivativesxe2x80x9d, Verlag Chemie, 1971). FIG. 1 shows the equilibrium which exists between the monomeric form of lactic acid and the oligomers over the whole range of conceivable concentrations.
The condensation or oligomerization reactions of lactic acid in fact correspond to esterification reactions; they are catalysed by Brxc3x6nsted and Lewis acids and bases. Consequently, in order to avoid or minimize the occurrence of these reactions, it is imperative to carry out at least one preliminary purification step for removing any traces of impurities which would be capable of catalysing the oligomerization. Furthermore, it is also recognized that temperature accelerates the formation of oligomers (Holten C. H., xe2x80x9cLactic acid: Properties and chemistry of lactic acid and derivativesxe2x80x9d, Verlag Chemie, 1971). This explains why lactic acid in aqueous solution has for a long time been regarded as a substance which is not particularly volatile and which cannot be distilled at 100xc2x0 C. In actual fact, lactic acid condenses to form oligomers whose boiling point under atmospheric pressure is higher than 100xc2x0 C. More recent studies on the distillation of lactic acid by steam distilling at 160-200xc2x0 C. show that the substance can be distilled with yields which are of the order of 75 to 85%. However, these drastic conditions are detrimental to the quality of the product; it is impossible to avoid degradation and racemization. Noerdlinger (U.S. Pat. No. 924,494, DE 221,786 and DE 224,664) has proposed a variant of distilling by steam distillation. This technique consists in passing air or warm inert gas at high speed over the surface of a solution of lactic acid which has previously been freed of the water which it contains. However, the energy consumption and the low yields obtained result in the procedure being of low viability from the industrial point of view. For the sake of completeness, we may also point out that there have been reports of other equipment and arrangement profiles which make it possible, with a greater or lesser degree of success, to concentrate and distil lactic acid, which is in dilute solution in water, under reduced pressure in an evaporator which has a vaporization surface which is very large as compared with the volume of liquid involved (Sepitka A., Prumisi Potravin 13, 385 and 605 (1962) and 14, 45 and 82 (1963); Shishkini A. V., Domanskii I. V., U.S.S.R. 709,613 (May 10, 1977)).
The present invention consists of a process for purifying lactic acid in an aqueous solution, as obtained from a fermentation medium or any other source, which has been previously freed of solid compounds and/or biomass. The reader is referred to any method known to the skilled person, such as centrifugation, flocculation, microfiltration, etc., as far as the step of separating solid compounds is concerned. By contrast, the purification process described in this invention is original in the sense that it ensures that a very high quality of lactic acid is obtained, with a particularly high mass yield and a minimum consumption of energy. xe2x80x9cVery high qualityxe2x80x9d is understood as meaning residual concentrations of mineral and organic impurities which are such that the purified lactic acid can be used for pharmaceutical applications in accordance with any of the current Pharmacopoeias. Moreover, the lactic acid which is purified by the process described in the present invention is thermo-stable, that is it remains colourless after a 2-hour heat treatment at 180xc2x0 C. and retains the optical activity of the lactic acid employed (stereospecific process). xe2x80x9cMass yieldxe2x80x9d is understood as being the ratio, expressed in per cent, of the mass of purified lactic acid over the mass of lactic acid employed; these masses correspond to lactic acid concentrations of 100%. Applications which require lower purity can, of course, also be met by the technique which is proposed in the present invention. The quantitative and selective aspects of this purification process are ensured by jointly implementing (1) a pretreatment which is designed to remove substances which are capable of catalysing the lactic acid condensation reaction, (2) temperature, dwell-time and viscosity conditions which reduce the involvement of the same condensation reactions, and (3) temperature, dwell-time, viscosity, pressure and equipment profile conditions which make it possible to concentrate the lactic acid so as to achieve a concentration by weight of 100%, and to distil it.
The present invention describes a process for purifying lactic acid which results from an aqueous solution of this acid, such as obtained from a fermentation medium or any other source which has been previously freed of the solid substances and/or biomass which may possibly have been present. FIG. 2 illustrates the lactic acid purification process to which the present invention refers. This process essentially comprises the following steps:
1. Pretreatment of the Dilute Solution of Lactic Acid (1)
The pretreatment under consideration within the context of the invention consists in removing ionic substances which are able to catalyse the condensation or oligomerization of the lactic acid. This pretreatment is carried out at a low concentration of lactic acid, that is at a concentration less than 80%, preferably less than 50% and still more preferably less than 30%. An approach which is favoured by the present invention consists in using ion exchange resins to remove the ionic substances. Thus, bringing the lactic acid solution into contact with an anion exchange resin which has previously been conditioned in basic form (OHxe2x88x92) enables the anionic impurities contained in the treated solution to be exchanged for hydroxide groups. The present invention does not stop at using solid anion exchange resins, but comprises any other technique known to the skilled person which makes it possible to remove anionic loads in favour of hydroxide ions, such as using fatty amines which are quaternized and which are present in ammonium hydroxide form in solution in at least one in water non-miscible organic solvent. In this case, the anion/hydroxide exchange takes place at the interface of the non-miscible phases and is followed by the phases being separated. A preferred approach of the invention consists, prior to carrying out the anion exchange step, in carrying out a treatment which is characterized by the lactic acid solution being freed of the monovalent, divalent, trivalent and/or polyvalent cationic loads which may be present. The cationic impurities are removed by bringing the lactic acid solution into contact with a cation exchange resin which has previously been conditioned in acid medium (H+) . This approach is preferred insofar it avoids the formation and precipitation of metallic hydroxides, which are of low solubility in water, during the anionic treatment. Here, too, the invention is not limited to cation exchange resins but extends to any other technique known to the skilled person which is able to exchange the cations which are present in the lactic acid solution in favour of protons. The reader is referred, for example, to the use of a fatty acid of the carboxylic or sulphonic type which is dissolved in at least one in water non-miscible organic solvent. The cation/proton exchange takes place at the interface between non-miscible solvents and is followed by the phases being separated.
2. Concentration of the Lactic Acid Solution (2)
This step of the invention consists in concentrating, rapidly and at low temperature, the lactic acid solution, which has been previously treated by the method used as the first step of the invention (1), so as to achieve a concentration of between 50 and 90%, preferably between 70 and 90%. A preferred approach of the present invention envisages conducting this evaporation under reduced pressure, which is maintained between 50 and 500 mbar absolute, preferably between 50 and 250 mbar, in order to ensure that the solution boils at a temperature which is as low as possible. This step of the invention is carried out using any technique known to the skilled person, such as evaporation in a falling film.
3. Post-concentration of the Lactic Acid Solution (3)
This step post-concentrates the solution issuing from the apparatus used for step (2) up to a lactic acid concentration of 100%. The procedure can advantageously be carried out, using a minimal dwell-time and at a temperature which is as low as possible, in a mechanically shaken thin-film evaporator or using a short-path evaporator. The pressure is of the order of from 10 to 500 mbar, preferably between 50 and 300 mbar, and, still more preferably, between 50 and 150 mbar. The temperature of the heating wall of the body of the evaporator is adjusted so as to support the vaporization of the free water contained in the solution to be concentrated without, for all that, overheating this latter solution; that is a temperature which is between 50 and 150xc2x0 C., preferably between 80 and 120xc2x0 C. Surprisingly, it has been observed that, if the lactic acid is present quantitatively in the form of the lactic acid monomer (and in the absence of free water, concentration=100%), it is possible to distil it under reduced pressure in a reactor which maximizes the vaporization surface in relation to the volume of liquid. As well as claiming the use of such a reactor profile for distilling the lactic acid, the present invention guarantees that this concentrated acid is obtained quantitatively, in the form of distillable monomer, prior to its actual purification by distillation.
4. Purification of the Lactic Acid by Distillation (4)
This step is characterized by the demineralized and concentrated lactic acid solution, as produced in steps (1) to (3), being subjected to conditions such that the monomer (and, to a lesser extent, the dimer) of this acid is/are distilled quantitatively and selectively. xe2x80x9cQuantitativelyxe2x80x9d is understood as meaning that the entirety of the distillable fraction is efficiently distilled. xe2x80x9cSelectivelyxe2x80x9d is understood as meaning that only the monomer (and to a lesser extent the dimer) of the lactic acid is/are distilled, without entraining impurities or degradation products. This step is advantageously conducted in a reactor which maximizes the vaporization surface in relation to the volume of liquid, that is by a reactor which exploits the properties of the thin film. A preferred approach of the present invention consists in using a mechanically shaken thin-film evaporator, on the outside of which the purified lactic acid is condensed, or a short-path evaporator, possessing an internal condenser, for distilling 100% lactic acid. The skilled person is familiar with the fact that such a system maximizes the heat-exchange surface and the vaporization surface. The temperature of the wall is maintained at between 50 and 180xc2x0 C., preferably between 80 and 160xc2x0 C., more preferably still between 110 and 160xc2x0 C. The pressure is between 10xe2x88x923 and 10+2 mbar absolute, preferably between 10xe2x88x921 and 2.10+1 mbar absolute, still more preferably between 1 and 10 mbar. A preferred approach of the present invention is that of arranging the evaporator vertically, enabling the film to move forward under the combined impetus of mechanical shaking and gravity. According to a variant, which represents an improvement, which is not essential for the present invention, the purification residue can be conducted towards a second still in which the temperature and pressure conditions are more drastic (FIG. 2, step 4a). The lactic acid which is supplied from this post-distillation, and which is partially purified, can be recycled either towards feeding the main still (step 4) or upstream of the process. A preferred version of the present invention is that of adding an additional substance which is intended to facilitate the thin-layer flow and evaporation of the lactic acid during the distillation and/or post-distillation step(s). This additional substance comprises any non-toxic substance which is chemically inert vis-xc3xa1-vis the lactic acid, which is of low volatility, which is thermostable and of low viscosity under the distillation and post-distillation conditions, and which is preferably non-miscible with the lactic acid in order to facilitate its separation by decantation and recycling. By way of example, we may mention that using paraffins such as Fina Vestan A80B, A180B and, preferably, A360B promoted the draining-off of impurities and evaporation of the lactic acid while at the same time meeting the abovementioned requirements. Other details and features of the invention, which are given below by way of non-limiting examples, are evident from the description, as are some possible forms of implementing it.